Optical devices for observing light of specific wavelength such as infrared light from an object have been used heretofore. Such optical devices are provided with a mechanism for suppressing influence of light from the part other than the object. For example, Patent Literature 1 below discloses an infrared detecting device having a toroidal warmshield comprising segments. This infrared detecting device is provided with a vacuum window in a front face of a coldshield surrounding an infrared detector, and three toroidal reflective members are arranged in front of this vacuum window. These toroidal reflective members have respective openings in their centers and these openings are arranged as aligned on a central axis so that they are located in symmetry with respect to the central axis of the detecting device. The sizes of the respective openings are set according to the diameter of an optical image to be detected by the infrared detector. Inside surfaces of these toroidal reflective members are toroidal surfaces.
Patent Literature 2 below discloses an infrared optical device for letting infrared light from an object pass through an interchangeable lens and impinge on a detector element, this infrared optical device is equipped with a Dewar vessel provided around the detector element and two mirror apertures provided outside the Dewar vessel, and these mirror apertures are arranged as movable along the optical axis of the interchangeable lens. Mirror surfaces are provided on the inside of the mirror apertures, the infrared light from the object travels through opening portions of the mirror apertures to reach the detector element in the Dewar vessel, and infrared light emanating from the part other than the object is prevented from impinging on the detector element, because only the cooled part or only the detector element is seen when the mirror surfaces are viewed from the detector element. As a result, good imaging performance can be achieved.